1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling one or more multiuser channel quality indicators (MU-CQIs) for multiuser multiple-input multiple-output (MU-MIMO) and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (COMP) transmission/reception, UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
In addition, multiuser multiple-input multiple-output (MU-MIMO) is currently discussed in the 3GPP. In detail, multiple transmit antennas are installed at the eNB, such that the eNB can operate the MU-MIMO by using the transmit antennas according to one or more precoding matrices. For example, the eNB may need accurate channel state information (CSI) to select a precoding matrix properly for one or more transmissions. In another example, a UE may feed back a channel quality indicator (CQI) and a preferred precoding matrix to the eNB, such that the eNB can select the precoding matrix according to the CQI and the preferred precoding matrix. Then, the eNB performs transmissions to multiple UEs via the same resource (e.g., the same subband(s) and/or the same time period (s)) by operating the MU-MIMO according to the precoding matrix. Thus, performance (e.g., throughputs) of the eNB and the UE is improved due to efficient usage of the resource.
However, methods of feeding back the CQI proposed for single user-MIMO (SU-MIMO) may not be suitable for the MU-MIMO, and methods of feeding back the CQI proposed for the MU-MIMO is still unknown. The eNB may not operate the MU-MIMO properly with the CQI fed back according to the SU-MIMO, and the performance of the eNB and the UE can not be improved greatly. Thus, feeding back the CQI for the MU-MIMO is a problem to be solved.